This invention relates in general to thermal insulating products and, more specifically, to high temperature flame resistant thermal insulation using a combination of polyimide foam and porous, lightweight inorganic particles in insulating products.
Lightweight, porous inorganic materials such as pumice, expanded vermiculite and "popped" perlite have long been used in thermal insulation, lightweight concrete, as packing material, etc. Pumice is a highly porous igneous rock, primarily an aluminum silicate. Perlite is a form of glassy rock, similar to obsidian, which when heated to its softening temperature rapidly expands or pops to form a fluffy lightweight material similar to pumice. Vermiculite is a mineral of the mica group, a hydrated magnesium-aluminum-silicte, having the ability to expand 6 to 20 times in volume when heated above 1400.degree. F. When used as thermal insulation, these materials are ordinarily used as loose particles to fill hollow walls or the like, or are incorporated in inorganic binders such as concrete. This insulation is inexpensive an fairly effective, but is not well adapted to complex light weight insulation shapes, such as pipe insulation;. Further, the inorganic binders have poor insulating qualities and are heavy.
Organic binders, such as epoxies, have been used in some case to reduce the weight and improve the formability of porous inorganic particle based insulating materials;. However, these organic binders tend to limit the temperatures at which the insulating materials can be used to less than 300.degree. F. due to binder degradation. Also, the insulating qualities of the binder itself tends to be low.
In low temperature applications, with complex insulation shapes, foamed organic resin insulation has been widely used. Typical of these are expanded cellular polystyrene as described by Charpentier in U. S. Pat. No 3,863,908, polyurethane foams as described by Willy in U.S. Pat. No. 3,998,884 and phenolics as descried by Bruning et al in U.S. Pat. No. 3,883,010. While these materials often have excellent insulating characteristics and are inexpensive to manufacture, they often have low strength and low impact resistance and cannot be used at temperatures above 300.degree. F. These organic materials degrade at higher temperatures, may burn, and often emit toxic gases at high temperatures or when exposed to a direct flame.
Recently, a number of polyimide foam insulating materials have been developed, such as those described by Long et al in U.S. Pat. No. 4,621,015 and Gagliani et al in U.S. Pat. Nos. 4,506,038 and 4,426,463. These polyimide materials have much greater resistance to high temperatures, resist burning and degradation when exposed to direct flames and do not emit toxic gases at high temperatures.
Additives, such as fibers, talc and microballoons may be added to the foam material, primarily to improve strength. Such polyimide materials, however, tend to be expensive, require high temperatures for foaming and curing and tend to have lower melting points than vermiculite which melts at a temperature in excess of 2000.degree. F.
Thus, there is a continuing need for improved thermal insulating materials which combine ease of forming and high temperature resistance with low material and manufacturing costs.